1. Field of the Invention
The present invention generally relates to noise canceling methods and apparatuses and, more particularly, to a noise canceling method and apparatus for accurately extracting signals buried in noises in communications under a large noise circumstance.
Such a noise canceling method and apparatus is preferably applicable to apparatuses over various fields as mentioned below.                a modem for power-line carrier communication for achieving fast data transmission using an electric line such as a power line having many noises;        a CATV modem, an ADSL modem, a VDSL modem, a radio LAN of 2.4 GHz band, other radio transmission fields and an optical transmission field;        a magnetic disk drive units or optical disk drive unit for achieving achieve high-density recording by enabling an extraction of signals which tend to be buried in noise under a high-speed operation;        a semiconductor device using a fast multi-value transmission technology; and        a demodulation device of a voice recognition apparatus, an image compression apparatus and a barcode scanner under a noisy circumstance.        
2. Description of the Related Art
Although a description will be given of an example in which the present invention is applied to a modem for power-line carrier communication as an apparatus used under a high-noise circumference, the present invention is also applicable to an apparatus of various fields.
As shown in FIG. 1, a power of a power supply transformer station 24-1 is supplied to a pole transformer 24-3 via a high-voltage line 24-2 of 6.6 kV, and further supplied to a house 24-6 via a low-voltage line 24-4 of 100 V/200 V and a branch line 24-5.
When a power-line carrier communication is performed in the above-mentioned power line system, an optical fiber (not shown in the figure) is provided between an access node 24-11 of the power supply transformer station 24-1 and the modem provided to the pole transformer 24-2 in parallel to the high-voltage line 24-2 so as to transmit the communication signals via the optical fiber. The communication signals are transmitted between the pole transformer 24-3 and the house 24-6 via the 100 V/200 V low-voltage line 24-4 between a modem, which is plugged into a receptacle connected to interior wiring 24-7 within the house 24-6 and a modem of the pole transformer 24-3.
The above-mentioned power line system is considered The low-voltage line 24-4 is considered to be an inductor of 1 μH/m as shown in FIG. 2B with respect to a spectrum of a transmission signal TX from the modem of the pole transformer 24-3. If the length of the low-voltage line is 150 m, it is considered to be an inductor of 150 μH/m. Additionally, the branch line 14-5 connected to the low-voltage line 24-4 is considered as a capacitor of 75 pF/m. If 50-m branch lines corresponding to 30 houses are connected to the interior wiring 24-7, the branch lines are considered as a capacitor of 0.1125 μF. In addition, each of various electrical appliances connected to the interior wiring 24-7 is provided with a radio interference suppression capacitor between the AC 100 V lines, which provides a large capacitive load.
As a result, when viewing the low-voltage line from the pole transformer 24-3, the low-voltage line is considered as a low-pass filter (LPF) of a low-pass type as shown in FIG. 2B, and a high-frequency component of the reception signal RX greatly attenuates as shown in FIG. 2C. Accordingly, in worst case, the high-frequency component may be buried in noises N when the high-frequency component reaches a communication terminal in the house.
On the other hand, although the low-frequency band component does not attenuate as much as the high-frequency band component, the low-frequency band component is also buried in a large noise N as shown in FIG. 2C since an extremely large amount of random noise (white noise) is emitted from electric appliances using a switching power supply or an inverter device. Accordingly, a fast data communication using a power-line carrier has not been put into practical use, and there has been a demand for finding a solution for a long time.
As a solution of such an issue, various modulation method such as an FM modulation method, an FSK modulation methods or a PSk modulation method, which are said to be resistant to a noise, has been used as a modulation method of a modem for the power-line carrier communication. However, a limited application has been made in practice, such as an application related to a low data transmission speed of less than 1200 bps since the power line has an extremely large noise level.
Additionally, although an attempt has been made to put the power-line carrier communication using a spread spectrum method in practical use, a transmission capacity sharply decreases when an S/N value is a minus value as shown in FIG. 2C under a white noise circumstance. The transmission rate is 100 kbps at maximum, and it becomes incapable of carrying out communication in worst case. Further, an orthogonal frequency division multiplexing (OFDM) method using a multi-carrier modulation method has been introduced so as to attempt an application of a technology to carry out communication while avoiding a carrier band having a lot of noises.
However, the number of switching power source and inverter devices in home electric appliances, which are main source of noises, tends to increase, and it has become further difficult to avoid an attenuation of high-band signals due to the capacitive load. Accordingly, the power-line carrier communication can be put in a practical in a low-speed communication, but it is impossible to achieve the power-line carrier communication at a high rate as high as a few Mbps.
In the future, the number of switching power source and inverter devices in home electric appliances will be increased, and it will become further difficult to avoid an attenuation of high-band signals due to the capacitive load. Under such circumstances, the communication being carried out while avoiding noises in the prior art is not sufficient. Rather, it is effective to positively take measures for noise so as to achieve a fast communication by canceling or removing the noise.
The applicants discloses, in Japanese Patent Application No. 2000-359949 titled “noise canceling method and apparatus”, the invention to achieve a fast communication by reproducing reception signals buried in noises, if the S/N value is minus as shown in FIG. 2C, by canceling a low-band noise component having a small signal attenuation. The invention of the prior application extracts reception signals buried in noises by turning the S/N value to a plus value by removing a noise component in a dominant band with respect to a colored noise in a macroscopic view. The band of noise to be canceled is fixed within a predetermined range, and it is effective to a case in which the noise component is particularly concentrated into a low band.
However, there is a case in which a noise having a comb-like spectrum distributed in a wide band is generated due to a noise of use of an inverter provided equipped in home electric appliances or a frequent band noise generated by radio interference by AM broadcasting carrier. In such as case, if only a noise within a predetermined range is cancelled, the S/N value deteriorates which generates frequent reception errors since noises in ranges other than the noise canceling band remain uncanceled.